Bcl-2-associated athanogene 3 (BAG3) is a member of a conserved family of cyto-protective molecular co- chaperones. A genome-wide association study, involving multiple families with heart failure due to dilated cardiomyopathy (DCM), identified associated mutations in BAG3. In the same study, analysis of a large four- generation family identified a single amino acid substitution of glutamic acid 455 to lysine (E455K) in BAG3 as the unequivocal cause of DCM. Several other human genetic studies have also identified other mutations in BAG3 that are associated with skeletal and cardiac myopathies. Two independent groups have reported that mice global null for BAG3 exhibit impaired postnatal growth and early lethality at 3-4 weeks. One group reported skeletal myopathy with disrupted Z-disks, characterized by non-inflammatory myofibrillar degeneration with apoptotic features in the knockout mice. The other group reported no prominent apoptosis in these mice but thick and disrupted Z-disks in skeletal muscle. Premature lethality of these mice prevented study of the role of BAG3 in adult heart function. In addition, little is known as to specific roles of the co-chaperone protein BAG3 in cardiomyocytes, or mechanisms by which the BAG3 E455K missense mutation leads to the progression of cardiomyopathy. To address these issues, we have generated BAG3 global and cardiac- specific knockout, and BAG3 E455K knock-in mouse models. Our preliminary data revealed that cardiac- specific knockout of BAG3 results in DCM. Intriguingly, the heat shock protein (HSP), HSPB8, a known partner of BAG3, is dramatically down regulated in hearts of cardiac-specific BAG3 mutants. Homozygous BAG3 E455K mutant mice have a lethal phenotype, similar to that of BAG3 global knockout mice. We have also found that E455K mutant BAG3 protein has lost the ability to interact with HSP70. Taken together, the foregoing observations indicate that BAG3 and its associated chaperone complex play key roles in maintaining normal cardiomyocyte and whole heart function. Accordingly, our hypothesis is that BAG3 plays an essential role in cardiomyocyte protein homeostasis and cardiac function by forming a complex with chaperone proteins to regulate protein quality control, and that the E455K mutation in BAG3 impairs specific aspects of BAG3 function to lead to cardiomyopathy. Our Specific Aims are: 1. To characterize the roles of BAG3 in cardiomyocytes by analyzing BAG3 cardiac-specific knockout mice and littermate controls for protein homeostasis, cardiac function, and the progression of DCM; 2. To elucidate mechanisms underlying cardiomyopathy consequent to the E455K mutation of BAG3 by analysis of BAG3 E455K knock-in mice, comparing results with BAG3 global loss-of-function mice; 3. To investigate mechanisms by which the BAG3 E455K mutation impacts human cardiomyocyte function, utilizing gene targeted human embryonic stem cell- derived cardiomyocytes containing BAG3 E455K or BAG3 loss-of-function mutations.